4 Answers
4

Generally speaking, I would write earth when referring to soil, the ground, or land as opposed to sea. I would capitalize Earth when referring to the planet (and even more specifically, our planet; there are many earths out there). In short: if it's a proper noun, it should be capitalized.

A handful of earth.

The earth under this house.

The earth beneath my feet.

I agree that earth should not be capitalized here. However, I could come up with contexts in which I would capitalize it even in these sentences, though they would be rather poetic or sci-fi (read: far-fetched).

4. What on earth?

I would capitalize Earth here. To me, this question is equivalent to "What on this planet?", "What in this world?", "What on Mother Earth?", and not to "What on soil?" or "What on land?". (If you want a comprehensive, highly scientific study, then I am happy to report that out of the first 30 Google results for "What on Earth", Earth is capitalized in 25 cases.)

5. The heavens above and the earth below.

Soil, ground, land as opposed to the sky. No need to capitalize.

Edit: as Neil Fein points out in the comments, this could refer to the planet in certain contexts, in which case I would capitalize it (cf. 1, 2, and 3).

6. The earth moved.

It depends. Did the Earth (proper noun) move around the Sun (proper noun), or did the earth (soil, ground) move because there was an earthquake?

I have one nit to pick, with #5: "The heavens above and the earth below." While in most cases I would agree with you that this refers to the ground beneath one's feet, I would capitalize if the context made it clear that this were referring to the planet. (The same logic you used for #1, 2, and 3.)
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Neil FeinAug 31 '10 at 22:34

1

@Neil: absolutely, I was actually thinking about that for quite a while. But that also got me thinking whether we should also capitalize Heavens, and I didn't want to open that can of worms. It's late at night and I can't think clearly.
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RegDwigнt♦Aug 31 '10 at 22:40

How about capitalizing Earth (the planet) when there's no article and not capitalizing it when there is? Thus: The inner planets include Mercury, Venus, Earth, and Mars. And: The satellite was returned to the earth on the space shuttle.
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moiociSep 1 '10 at 2:13

@moioci: "...and this computer shall be called... THE Earth." That's not bad, but it doen't hold up. Think about "digging a hole in the earth"; I think this has to be a context-based decision.
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Neil FeinSep 1 '10 at 7:04

@RegDwight: Insomnia and StackExchange don't mix well. I've found that out as well!
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Neil FeinSep 1 '10 at 7:05

I would write Earth when referring to the planet where we live because it is a proper noun (like Mars, and Venus, for example).

Looking at the New Oxford American Dictionary, it's reported that

The earth is the third planet from the sun in the solar system, orbiting between Venus and Mars at an average distance of 90 million miles (149.6 million km) from the sun, and has one natural satellite, the moon.

The definition for earth as a noun starts with

(also Earth) the planet on which we live; the world: the diversity of life on earth.

As for the example phrases you wrote, I would write them as

A handful of earth.
The earth under this house.
The earth beneath my feet.
What on earth?
The Heavens above and the Earth below.
The earth moved.
We returned quickly to Earth.

I wrote earth on the fourth phrase because it's a way of saying, which has a different meaning from What is happening on Earth? I am actually not 100% sure on how I would wrote earth in that case.
In the earth moved I wrote earth because the word is not referring to the planet (I suppose it is the sentence you say when there have been a quake).

Can you clarify which you would capitalise then, as I did with my lower/upper/unsure note? I think I understand and agree with this answer but think that 5 could be both?
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Neil TroddenAug 31 '10 at 21:51